It is known in the art to measure the stiffness of timber pieces in order to perform non-destructive stress grading thereof based on the measured stiffness and correlated strength of each one of the timber pieces.
Known methods to measure the stiffness of timber pieces commonly include taking a number of measurements of stiffness over a length of a piece of timber (for example, on its side in the case of a plank). In order to perform such measurements, each one of the pieces of timber can be moved either longitudinally or transversally, with supports for supporting a section of the piece of timber in place and a load to apply a force on another section of the piece of timber, between the supports.
Canadian patent application no. 2,363,092 discloses an apparatus for grading timber pieces, where measuring of the stiffness of each piece of timber is performed by applying a force to the piece of timber, at a point intermediate its length, as the piece of timber is supported at or near the ends thereof, without moving the piece of timber lengthwise. In order to measure the stiffness of the piece of timber, the patent application describes that either a predetermined force can be applied by a ram and the resulting deflection of the piece of timber measured, or the force required to deflect the piece of timber by a predetermined amount can be measured. The document discloses that a single measurement can be taken by, for example, applying a force and measuring the resulting deflection of the piece of timber, or two or more measurements can be taken by applying a first predetermined force and measuring the resulting deflection and subsequently applying, for example, a second higher force and measuring the resulting deflection.
The above described methods and apparatuses to grade pieces of timber using measurement of a stiffness thereof however suffer from several drawbacks. Indeed, in order to measure the stiffness of the timber pieces using such methods and apparatuses, a dedicated machine must be added to the production line, which requires additional space and increases the time required for a piece of timber to travel along the production line. Moreover, given that pieces of timber are not all identical, the measurements obtained for a piece of timber can be biased by the natural curvature of the piece of timber and/or by imperfections (tears, holes, slits or the like) on the surface from which the measurements are taken.
In view of the above, there is a need for an improved mechanism for measuring the stiffness of a piece of timber which would be able to overcome or at least minimize some of the above-discussed prior art concerns.